1. Field of the Invention
This invention relates to thermal transfer materials and, more particularly, to a dry thermal interface material, and related methods of producing and applying the material between an electronic component and a heat sink.
2. Description of the Related Art
Electronic assemblies are usually fabricated with a plurality of electronic components attached to a substrate, such as a printer circuit board. In order for these assemblies to operate properly and reliably for extended periods of time, the heat generated by the components must be efficiently and reliably transferred from the component to the board, which acts as a heat sink.
Such electronic assemblies are operating at increasingly higher temperatures as they are built smaller and run faster. With smaller electronic components, the density can also be increased, which further increases the need for efficient and reliable removal of heat.
The ultimate theoretical thermal transfer occurs where a component and the heat sink interface in continuous contact. In reality, however, the respective surfaces of the component and heat sink have irregularities, such as microscopic voids or pores, which trap air. Since air is a poor conductor of heat, these irregularities/voids must be filled with some thermally conductive material, to effect more efficient thermal transfer. The following materials and techniques have been used to promote this thermal transfer.
Silicone-based thermal grease served as an early thermal interface material for electronic assemblies. Such grease is formed by dispersing thermally conductive ceramic fillers in silicone to form a sticky paste.
When the grease is applied between a surface of the electronic component and a surface of the heat sink, the grease fills the voids and eliminates the interstitial air. Any excess grease flows out at the edges of the component. The use of this grease allows for the thinnest possible joint as both mating surfaces come into contact at their high points, resulting in a very low thermal resistance.
Although such grease has proved to be a very good thermal conductor, problems are associated with its use. It is messy, due to its moist-to-the-touch, sticky state, and it is time-consuming to apply (e.g., generally the right amount of grease must be applied). Also, if the grease is applied to a protective sheet liner, to facilitate handling, shipping, etc., when the liner is removed prior to application of the grease to the electronic component surface, up to 50% of the grease may remain on the liner, causing waste, increasing costs, and resulting in a less effective thermal interface than desired. In addition, during operation of the electronics, when heat is being generated, the thermal grease migrates away from the area of application. Also, silicon-based greases exhibit the disadvantage of causing silicone contamination of a wave solder bath. If silicone oil migrates onto a printed circuit board, any solder re-work on the board will not adhere. Such migration may also cause short circuits on the board.
Non-silicone thermal grease was then developed to address many of the above-discussed problems associated with silicone-based products. Non-silicone greases are formed by dispersing the thermally conductive ceramic fillers in hydrocarbon oils.
While the non-silicone-based greases addressed the migration/contamination characteristics of silicone-based products, they still suffered from being messy, since they still exhibited moist/sticky characteristics, and they were still difficult and time-consuming to apply.
In a further effort to provide an acceptable replacement for thermal grease, relatively thicker and drier elastomeric thermal pads were developed. Their composition is basically silicone rubber-containing heat-conducting particles, such as zinc oxide, aluminum oxide, aluminum nitride, and boron nitride. The advantages of using these pads have included the facts that they are less messy (due to being drier), installation is easier and less time-consuming, and they eliminate the need to apply only the correct amount of grease with each application.
As noted above, however, the ultimate thermal interface is where two parts touch at as many points as possible, and only where microscopic voids appear, are they filled. Whereas the above-described grease flows easily into these voids, and is easily displaced to allow as much direct contact as possible between the component and the heat sink, these pads do not allow for any direct contact between the surfaces of the component and the heat sink. That is, these silicone elastomers deform to surface irregularities only when a significant compressive load is applied, which may be detrimental to the electronic component. At low pressure, the pad simply cannot fill the air voids between the surfaces, causing a relatively very high thermal resistance.
Wax or paraffin-based phase-change materials have also been developed, which exhibit grease-like thermal performance and, due to their relative dryness, exhibit easier elastomeric pad-like handling and installation. These phase-change materials have been used in a stand-alone form, have been reinforced with fiberglass, or have been coated onto foil or Kapton®. Kapton is a thermally conductive but electrically insulative polymide film available from the DuPont company. These phase-change materials are solid at room temperature, but they behave much like thermal pastes or greases once they reach their phase-change, or melt operational temperature, i.e., usually between 40° C. and 70° C.
Since these phase-change materials are solid and dry at room temperature, they are not messy to apply. As they are heated they become liquid and flow into the pores. However, in a vertical orientation of the electronic component, they will flow out of the interface, again leaving voids. These materials require pressure sensitive adhesives to adhere to parts during assembly, which adhesives undesirably increase thermal resistance. The operational high temperature range for phase-change materials is only 150° C., however, versus 200° C. for thermal grease. Further, in a “cold plate” application, i.e., using water and/or thermoelectric modules to help cool electronic assemblies, the temperature would not reach the melt operational temperature, so the phase-change material would not receive enough heat to melt into place (wet the surface), and therefore would not be useable, whereas grease works at such temperature. Further, each thermal cycle and subsequent phase-change may introduce new air voids that may not be refilled.
In light of the above, thermal pads are easy to use, but exhibit a relatively high thermal resistance. And, while phase-change materials may outperform pads in terms of thermal transfer efficiency, they still bear limitations in use and performance. Thermal grease offers superior performance to these grease replacements, including most particularly the lowest thermal resistance, but can be very messy and labor-intensive during application.
Although the prior art described above eliminates some of the problems inherent in the thermal transfer art, this prior art still does not disclose or teach the most efficient compound and related methods of production and use.